Since the year 2000, the AIS (Automatic Identification System) has been stipulated as an obligatory standard by the International Maritime Organization (IMO) in order to increase the safety of international shipping traffic. This locally bounded radio system is used in this case for the interchange of navigation and other shipping data that are meant to allow the ships to obtain a comprehensive overview of the adjacent shipping traffic. The primary aim in this case is to avoid collisions between ships.
The individual ships use an AIS transponder arranged on the ships to transit their own shipping data using the broadcast method, said shipping data then being able to be received by the adjacent ships. The reception of shipping data from various ships in the surroundings of a ship therefore allows an overview of the adjacent shipping traffic to be derived, so that, by way of example, a ship is able to establish in what direction and at what speed adjacent ships, which are invisible on account of poor visibility conditions, for example, are traveling.
AIS is designed such that it alternately transmits on two channels in the VHF marine radio band, namely firstly on 161.975 MHz and secondly on 162.025 MHz. In this case, the individual AIS shipping data items are transmitted in fixed time frames, the use of which is automatically coordinated by the relevant subscribers (what is known as SOTDMA: Self organized time division multiple access).
Hence, there are just 2250 time slots per minute available to the individual subscribers for transmitting data.
In this case, the radio range of AIS from ship to ship is approximately 20 to 30 nautical miles, which corresponds to little more than normal visibility on the high sea. Although coastal stations can cover a radius of up to 100 km on account of their relatively high position, such coastal stations can be found only in highly frequented areas, in which maintaining such a coastal station is worthwhile from economic aspects.
On the basis of the limited visibility and also the SOTDMA transmission protocol that is used, those ships that are able to see and receive one another form an AIS radio cell within which the subscribers can send and receive without collision. In this case, the subscribers in the AIS radio cell negotiate their use of the available time slots so that collision-free interchange of the data is made possible. Hence, the AIS is merely a local radio system that, although it provides sufficient data for a ship on the high sea, appears to be little suited as a superordinate communication means.
Accordingly, AIS coastal stations can be used to transmit messages to the ships using AIS radio signals only within a short coastal range. Outside these regions or in areas in which such AIS coastal stations are not worthwhile on a geopolitical basis or for economic reasons, transmission of messages to the ships is possible only to a limited extent. Thus, although some ships can receive data using communication means, such as Iridium or INMARSAT C, both the purchase of these communication appliances and the use thereof are very costly, which means that this communication path is suitable only for emergencies.